Power adapters, chargers or power supply device are indispensable electronic devices for many kinds of electrical apparatuses and information apparatuses. Referring to FIG. 1(a), a schematic cross-sectional view of a conventional power adapter is shown. This power adapter 1 includes an upper housing 11 and a lower housing 12. A space is defined between the upper housing 11 and the lower housing 12 for accommodating a printed circuit board 13 therein. An AC inlet 14 and a DC power cord connector 15 are mounted on opposite sides of the printed circuit board 13. Several protrusion pieces 120 are integrally formed on opposite sides of the inner surface of the lower housing 12 for supporting the periphery of the printed circuit board 13. Many electronic components required for implementing functions of the power adapter are also mounted on the printed circuit board 13. Generally, depending on the weights, these electronic components are classified into two types: heavier components and lighter components. For neat drawings, however, only two electronic components 16 and 17 indicating heavier and lighter components, respectively, are shown in the drawing. Examples of these electronic components include but are not limited to transformers, metal-oxide-semiconductor field effect transistors (MOSFETs), bare dice, diodes, inductors, chokes, wound toroids, capacitors or resistors. As known, among these electronic components, transformers, inductors, chokes, wound toroids are relatively heavier than others.
As shown in FIG. 1(a), since the periphery of the printed circuit board 13 is supported on the protrusion pieces 120, a gap g1 is formed between the printed circuit board 13 and the inner surface of the lower housing 12. The gap g1 is advantageous to prevent direct contact of the soldering portions (not shown) of the components mount on bottom surface of the printed circuit board 13 with the lower housing 12 so as to assure good soldering properties. Otherwise, existence of the gap g1 still has some drawbacks. For example, in a case that the power adapter 1 accidentally falls down, the printed circuit board 13 may vibrate up and down in response to the sudden impact. Please refer to FIG. 1(b). An amplitude distribution analysis shows a two-node vibration spectrum. The largest vibration amplitude predominates at the region under the heavier component 16. In contrast, the region under the lighter component 17 exhibits less vibration amplitude. The nodes are substantially corresponding to the protrusion pieces 120. Accordingly, it is necessary to reduce the vibration amplitude at the region under the heavier component 16. If the vibration amplitude exceeds an allowable level, a fatigue crack of the printed circuit board 13 occurs.
Consequently, because of the technical defects of described above, the applicant keeps on carving unflaggingly through wholehearted experience and research to develop the present invention, which can effectively overcome the problems described above.